1. Field of the Invention
This invention relates to microwave switches and more specifically to three dimensional (3D) microwave switches, particularly tetrahedral or octahedral shaped T-switches, for routing microwave signals along selectable signal paths between a plurality of switch ports.
2. Description of the Related Art
Microwave switches are used in redundant switching networks on board spacecraft to route M input signals to M outputs through N (N&gt;M) failure-prone devices such as traveling wave tube amplifiers (TWTAS) to significantly enhance the networks' end-of-life reliability. This is accomplished using two layers of microwave switches, with each layer including M serial connected 4-port switches, for example, T-switches. The switches in the input layer are controlled to route the M input signals around the failed devices and through functioning devices. The switches in the output layer are controlled to route the signals produced by the M selected devices to the M outputs.
U.S. Pat. Nos. 4,070,637, 4,317,972, 5,063,364, 5,065,125 and 5,281,936 to Assal et al., Kjelbert, Tsoi, Thomson et al. and Cierzarek, respectively, show a known T-switch arrangement in which one of the ports is surrounded by the other three, and six microwave paths selectively interconnect the ports in a common plane. The T-switch has three different states, in which opposing pairs of the microwave paths are switched to a signal-conducting position to couple two pairs of ports while the remaining four paths are switched to a signal-attenuating position. Specifically, in the first state ports 1 and 2 are connected and ports 3 and 4 are connected. In the second state, ports 1 and 3 are connected and ports 2 and 4 are connected. In the third state, ports 1 and 4 are connected and ports 2 and 3 are connected. The multistate T-switch provides the flexibility required to reroute microwave signals in a redundant switching network.
In the known T-switches, the ports are typically coaxial connectors having outer shields that are grounded to an RF cavity and center conductors that are inserted into the cavity. The cavity is constructed with six waveguides that lie in the common plane between the connectors' center conductors. Each of the waveguides contains a conductive reed which is moved by an actuator between a signal-attenuating position abutting the waveguide's interior surface and a signal-conducting position coaxial with the waveguide and abutting the ends of the center conductors at each end of the waveguide. Because the microwave paths lie in a common plane, it is relatively simple to machine the cavity to align the coaxial connector' center conductors over the ends of the reeds and to control their height so that the reeds make proper contact.
The T-switches use a variety of different actuators to move the reeds. One conventional actuator includes a pivotable armature that pivots about the end of a permanent magnetic in response to pulses applied to a pair of electromagnetics. One end of the actuator moves a reed via a dielectric post. Tsoi uses a circularly shaped actuator that has one or more ridges and one or more indentations. When the actuator is rotated, the ridges depress a pair of reeds contacting them between the center conductors and the indentations release the remaining spring-loaded reeds so that they abut the waveguide's interior surface. Thomson et al. includes a rotatable armature that is driven by a stepper motor. The rotatable armature carries a plurality of permanent magnets which have predetermined polarities and each reed carries a permanent magnet. The reeds are selectively positioned in the waveguide by rotating the armature to place a permanent magnet adjacent the reed magnet to either attract or repel the reed. Cierzarek employs three cantilever leaf spring actuators, which are respectively displaced by the rocking action of a wobble plate caused by the repelling and attraction forced provided by a series of spaced magnetic coils. Rocking of the wobble plate to one of three selected positions displaces a particular leaf spring which in turn depresses a pair of selected reeds into bridging contact with the center conductors.
Although the known planar T-switch configurations are used effectively in redundant switching networks on board spacecraft, there are a number of aspects that bear improvement. A typical spacecraft may employ several hundred microwave switches so that a small reduction in the weight of each switch can amount to a significant cost savings. The actuators are the primary weight components of the switches, and thus a switch topology that would facilitate a simpler and lighter weight actuator is highly desirable. Second, in the planar topology the three inner and outer waveguides necessarily have different lengths. As a result, the signal paths through different ports have different microwave properties, which prohibits the overall system from being optimized. Third, the ends of the center conductors are flared substantially to ensure contact to the underlying conductive reeds. This limits the high frequency performance of the switch. Fourth, the physical access to the coaxial connectors is limited. Lastly, as the complexity of the redundant switching networks increases, it will be very difficult to develop planar microwave switches with enough ports to reroute the signals.